High-voltage direct current transmission enables high levels of electrical power to be transmitted over great distances with lower losses than is the case with alternating-current transmission systems, due to the fact that reactive power losses in alternating-current transmission systems become an increasingly important factor as the transmission path for the electric power becomes longer. For this reason, direct current transmission has technical advantages when given the same voltage but relatively great distances.
One- or multipart post insulators, i.e. those made of a plurality of individual insulators, have been used for decades to mount busbars or stranded conductors of an HVDCT system that are frequently at a height of 8 m. These previous post insulators are characterized by a ceramic solid core in order to withstand the high mechanical stresses, in particular, bending moments that may occur. Post insulators of this type that are provided with a solid ceramic core have been disclosed, for example, in CH 232740 or DE 1 035 719.
More recent developments, on the other hand, generally relate to hollow composite insulators, also usable as post insulators, that are made of glass-fiber-reinforced epoxy resin and include a shielding of silicone in which top and bottom ends are formed by metallic flanges, for example of aluminum. A method of making a composite insulator of this kind is disclosed in EP 1,091,365.
Due to its electrically insulating properties, the empty internal space of these composite insulators is filled, in particular, with sulfur hexafluoride, an inorganic chemical compound of the elements sulfur and fluorine with the molecular formula SF6. However, it must also be stated that any other insulating gas, such as, for example, nitrogen, can be considered for use here. Under normal conditions SF6 is a colorless, odorless gas that is noncombustible and is extremely inert, as is nitrogen similarly. It is a commonly found insulating gas for use in medium- and high-voltage engineering due to its high density, high ionization energy, and the property of binding free electrons.
The empty internal space of the composite insulators must be provided with an absolute seal relative to the outside atmosphere in order to ensure the effectiveness of the employed insulating gas over the full life of the composite insulator. To this end, each individual gas space is monitored in terms of its pressure conditions. A connection point for a protective monitoring device composed of at least one pressure sensor is provided for this purpose on the bottom flanges of the composite insulators in order to obtain information from the actual prevailing pressure conditions about the fill level or status of the empty internal space, which is filled with insulating gas, of the composite insulator.
This type of interior space monitoring entails a significant servicing cost, particularly in the case of multipart post insulators made of composite materials, i.e. with those that are assembled out of a plurality of separate hollow insulators to form a common post. If, for example, it is not the lowest but instead one of the following composite insulators of the multipart post insulator that must be inspected, the relevant stranded conductor or relevant busbar of the corresponding post insulator must be disconnect or deenergized by the monitoring device while the actual inspection is being performed. A further fundamental disadvantage of multipart post insulators is the separate monitoring of each individual post insulator.